Transducers, particularly accelerometers, are used in multi-channel acquisition of test data. In the fields of mechanical machine monitoring, industrial measurement and control, structural monitoring, vehicular testing and monitoring, wind loading, collisions, construction, shock sensing, earth movement, position sensitive manufacturing and assembly, plus many other applications, all require measurement of proper acceleration, usually at multiple locations. Among the most sensitive accelerometers are piezoelectric sensors that are able to translate changing force, particularly impacts, and the resulting mechanical motion or vibration into electrical signals. Another type of accelerometer, perhaps less sensitive, usually manufactured as an integrated circuit, is the MEMS type that consists of a cantilevered beam moving in an enclosure. Two beams in different planes can report two-dimensional motion. The website http://arduino.cc/en/Tutorial/Memsic2125 shows a chip accelerometer with a circuit that includes an analog to digital conversion circuit having two output data leads, X and Y reporting two dimensional accelerometer digital data to a circuit board associated with a computer for collection of serial data. The circuit arrangement, with the accompanying software programming, is suggestive of using a Universal Serial Bus.
The Universal Serial Bus, or USB, is an industry standard serial data communication interface between computers and peripheral devices. The standard does not specify the type of computers nor the peripheral devices, but is generally applicable to any. Since the invention of the standard over 15 years ago, data transfer rates contemplated by the standard have increased significantly, but the general theory of operation has remained the same. In USB communication, a controller in the host computer polls a bus for message traffic in serial format, including traffic from hubs. A USB hub is a device that expands a single USB port associated with a computer into many ports. Hubs can be stacked, one after another in geometric fashion resembling a tree, to gain a needed number of USB ports to a present maximum of 127 ports.
It is often desirable to transmit signals from sensors through USB ports and hubs. Analog signals of one or more sensors will usually go through certain pre-conditioning analog circuitry such as filters and amplifiers, and be converted into digital data by one or more A/D converters. The digital data from the A/D converter will be fed into a processor for further processing, display or storage through a USB port or hub.
When collecting transducer data in multiple channels using multiple USB ports, there is usually a need to synchronize the time of conversion of data for the A/D converters in order to compare and analyze the relationship between signals in each channel. In U.S. Pat. No. 8,412,975 P. Foster recognized the need for multi-channel USB data synchronization. A circuit was invented having a microcontroller for observing USB traffic and decoding from a USB data stream a periodic data structure, such as a clock carrier signal, containing information about a distributed clock frequency and phase. The circuit generated a software interrupt upon receipt of a predefined data packet, such as a start-of-frame (SOF) packet and for passing the software interrupt to the microcontroller. In turn, the microcontroller responds to the software interrupt by generating an output signal adapted to be used as a synchronization reference signal.
What is needed is an autonomous synchronizer for a transducer of accelerometer data that will allow simultaneous data conversion of multiple transducers sharing the same USB tree. The synchronizer must be accurate enough to assure that the multiple A/D converters convert the analog signals to corresponding digital signals simultaneously, particularly for shock and vibration data, but be sufficiently simple to allow integration with the transducer.